Chemical upgrading of chromium bearing materials



p 1959 D. L. HARRIS 2,905,546

CHEMICAL UPGRADING OF CHROMIUM BEARING MATERIALS Filed Deer 5, 1957 3 Sheets-Sheet 1 UPGRADING PROCESS Reducing Agent Chromate Ore or Concentrate Gas, Liquid or Solid 3 I L j Pulverize Pu/verize i i Mix Pelletize I alternate Grind alternate AC/d H 80 I Lea 'sh Final H= 2.0 to 5.0

Filter Residue Solution Cr/Fe 3.0 Cr/Fe 0.1

:EIl3- I00 C C cc e 8 E -80 5 Q 2 i n -60 .5 E 2 i X o 40 Precipitation of '3 Chromium from it} Le'ach Solution Vs. i. 20 i $1 i PH INVENTOR.

- I 1 Dwight L. Harris B pH of mean M AKZJ? 3.0 4.0 5.0 A%' :EIEiz Xxx/Z Sept. 22, 1959 D. L. HARRIS 5,

CHEMICAL UPGRADING OF CHROMIUM BEARING MATERIALS Filed Dec. 5, 1957 3 Sheets-Sheet 2 05 [I0 I15 20 2 5 3O Chromium iq Solution, gm. liter Retreatment of Leach With Roast to Precipitate -8.0 Chromium -70 Q g Q t nm/ w E a I No. 5 Leach,

/ Iror in Solution, gm. liter BIG-"E1 SINGLE STAGE LEACH Roasted Chromite Sulfuric Acid /-'20 I00 gm. 200 ml.

Cr Fe 1.6

@ .Spen'r Solution Cr/Fe Residue Dwight L. Harris Cr/Fe= 3.0' BY 27%qfl5 Cr Recovery 9996 7W p 1959 D. L. HARRIS 2,905,546

CHEMICAL UPGRADING OF CHROMIUM BEARING MATERIALS Filed Dec. 5, 1957 3 Sheets-Sheet 3 TWO STAGE LEACH Roasted Acid H 80 l-'l7 Chromafe 0.35 lb. H 80 per lb. Roast in Leach A Residue A Cr/ Fe 5.0

.835 lb. Cr 26.7% Solution Final pH =3.9

- Residue B Cr /Fe =2.9

.72 lb. Cr =25.9%

Waste Solution, high Fe,no Cr Cr/Fe 0.02 c

COUNTERCURRENT LEACH W/TH PH CONTROL Pregnonf .Solufion Cr/Fe 0.3

New Acid Roasted Residue Chromafe C Fe=4.0 Cr/Fe /.6

INVENTOR. Dwight L. Harris Spent Solution fjwqfih Cr/Fe 0.03 [VA/L194 United States Patent CHEMICAL UPGRADING OF CHROMIUM BEARING MATERIALS Dwight L. Harris, Nye, Mont., assignor to American Chrome Company, a corporation of Nevada Application December 5, 1957, Serial No. 700,874

13 Claims. (Cl. 75-1) This invention relates to the method of chemical upgrading of chromium-bearing ores and concentrates to obtain residues having higher chromium to. iron ratios than exist in the ore or concentrate.

Chromite ores and concentrates are the major source oi ferrochromium, chrome refractories and chromium ehemicals. today. The ratio of chromium to iron in ore and concentrates determines to a; large, extent the ratio of chromium to iron in the ferrochrorniumwhich is smelted from the material. Ferrochrome of various grades, is produced and generally the price varies with gradev and, is dependent, upon both percent chromium and the chromium to iron ratio. Many so-called low-grade chromite ores, having chromium to iron ratios less than 1L5 cannot be smelted. economically into acceptable grades of ferrochrome, since iron is reduced preferentially relative to chromium in the smelting process. If physical nce tr t o or h a up rading of th re an concen rat an increase the chrom m o i n a io t a u 3 o 1,, hi heg ade. fcrro hromi m can b smelted i e t y r m the mater al.

hrom te s a sp nel wh ch has. he. mpo ition. epresented y (M F r. Al e).z0l. T hemical compos tio o e pinei can, vary widely but for pro duetion of metal, particularly ferrochromium, the percents of chromium and iron are the important, analyses. Physical concentration of the chromite, mineral in a ch om tev r i u uallypcssibie. o a mi ed extent. but e us h chrom t mine al. c n a ns ir n. asv well as chromium. n e t c of h mineral, r n. in the min l tse f anno be emoved y physical meth dshemica Qt m t ng- P o sses. which. atta k. the lattice can separate the iron from the chromium.

It is the practice to physically concentrate Certain chromite ores into chromite concentrates having higher IiQa nt or me ti g re o a d. re ha ing high h o um t r n rat os e ou ht hr ughou thetworld. However, there are millions of tons of chromite ores t o hr mium o r n rati i h. m ght. be us f e rcchr me p o uc i it he pr ss f r p radi to a satisfactory ratio were economical.

It is therefore an object of this invention to provide a me the he i l pg ading of. chromiumrbearins ores and concentrates.

It is a further object of invention to provide an economical method for the upgrading, of low-grade h om um-b r ng, or s. at. s, res with low chromi m to iron ratios, which method enables. such ores tov be u e n err chr m produ i n- Fu her bje nd. advantage of. this, nvention, it not, specifically set forth, will become apparent during the course of the discussion which. follows.

Figure 1 is a flow sheet showing the entire process, which hereinafterwill be designated leaching with, pH control;

Eigure; 2, is a plot; of percent chromium precipitated against of the-,- leaching solution;

; Figure. 3, is= a graph showing the effect of-varying quan- 2 ties of number 5 leach, defined infra in the specification, upon the quantities of chromium and iron remaining in solution;

Figure 4 is a flow sheet showing the single stage leaching process of this invention (see Table I);

' Figure 5 is a flow sheet showing the two stage leaching process of this invention (see Table I); and

Figure 6 is a flow sheet showing the countercurrent leaching process of this invention (again see Table 1).

Generally, it has been found that chromite concentrate (made from chromite ore by any of several physical processes) can be upgraded by chemical means in a cone trolled process which includes the following steps:

(1) Grinding the chromium-containing material to fine size.

(2) Mixing the ground material with measured and controlled amounts of carbonaceous reducing materials such as coal, coke, char, or hydrocarbon gases, liquids or solids.

(3-) Heating to controlled temperatures ranging from 900 C. to 1300 C. dependent upon the nature of the reducing agent.

(4) Leaching the roasted material in sulfuric acid solution for various but controlled lengths of time and at various but controlled temperatures; also controlling the dissolution of the constituents by concentration of acid and pH control (5) Separation of the leach solution from the residue;

('6) Drying ofthe residue.

7 Pelletizing and /or sintering of the residueas: preparation for smelting.

More particularly, carrying out step 1 of the process, I prefer to grind the chromite material to approximately minus 325 mesh and the carbon material to approxi mately minus 100 mesh (Tyler Screen Series). Fine grinding of the chromite material is an advantage in obtaining better reaction in the reduction roasting operation. The degree to which the chromite burning material is ground is not a particularly important factor; In general, itmay be stated that the finer-the material is ground, the more complete is the reaction between the reducingagent and the chromite materialfor agiven temperature and length of time in the furnace. Thus, the size of the grindrecommended, 80-percent minus 325' meshfor-the chromite material, is a compromise between a finer size which might be desirable but which is also more costly as well as more difficult to handle mechanically, and a course grind of less trouble and expense. The carbon material need not be ground as fine as the chromite material, and the approximately 100 percent minus mesh is again not a particularly critical figure. It is more important that the denser, harder chromite ma terial'be ground toafi'nelydivided' state in order to obtain satisfactory reaction rates Generally, itmay be stated; that maxima for both the chromite and the carbon material sizes might beset at minus 8 mesh with a preference being expressed for finer sizes.

In step 2, it has been found advantageous, to use solid reducing agents such as coal and/or char in, preference tomethane or fuel oil. The; gaseous agents are more; reactive at low temperatures, but the products of such reduction are less amenable to separation of iron and chromium by simple leaching. T he chromium is more soluble after gaseous reduction than after reduction by coal, char, or coke. Coke (petroleum coke in particular) is less reactive at low temperatures and in fact requirestemperatures in the upper range, above 1200 C. for effective reaction. The product of the roast having coke presentmay be quite amenable to separation of" iron and chromium bysimple leaching in acid, but the-roast ing temperature isa physical limitation and the coke maybe more expensive than-the coal;

For the reasons given above, the examples following generally employ char or coke as the carbonaceous reducing agent, but it is well recognized that various liquid hydrocarbons may'be substituted for the solid forms of carbon in reduction roasting processes of this type. See, for example, U.S. Bureau of Mines RI 3847, Selective Reduction of Iron in Chromite by Methane-Hydrogen and Similar Gas- Mixtures, February 1946. Accordingly, wherever solid carbon in the form of graphite, coke or char (low temperature coke) is described herein, it is to be recognized that these may be replaced with suitable liquid or gaseous hydrocarbon materials. In step 3, it was pointed out that heating to between 900 C. and 1300" C., depending upon the nature of the reducing agent, is preferred. This temperature range is largely determined by economic considerations and accordingly certain higher and lower temperatures are possible. The upper limit is generally a mechanical limit set by limitations of the refractories in the furnace. The lower limit of operation is determined, to a large extent, by a rate of reaction which is slower with lower temperatures. Accordingly, even though a broader range may be postulated, the preferred range is the 900 C. to 1300 C. previously set forth.

I have found it advantageous to pelletize the mixed reducing agent and chromite material to get more effective reduction in the roasting process and also to decrease the dusting of the material as it is roasted. However, pelletizing is not an essential step in the invention. The pelletized, or unpellitized, material is roasted for two to three hours in a reducing atmosphere. If coal and/ or char are used, a temperature between 2000 F. and 2200 F, (about 1093 C. to 1205 C.) is preferred. Time and temperature and nature of charge (pellets or powder) are interrelated. Coke requires a higher temperature or longer roasting period. Heating the material in a muffie furnace with indirect heating in covered containers automatically provides a satisfactory atmosphere. From 0.15 to 0.25 lb. of coal Or char are used per lb. of chromite ore or concentrate. I

Sulfuric acid leaching of the roasted chromite ore or concentrate in various concentrations, not here limited but preferably dilute (1 to 4 up to 1 to 10 parts concentrated 96% H 50 acid by volume in water) and heating near boiling for one hour periods causes a large portion of the iron to be dissolved readily. A portion of the chromium also dissolves but the major portion of the chromium is left in the residue which contains other undissolved compounds having a content such as MgO, SiO A1 chromium, iron and carbon. The resulting residue with a high chromium to iron ratio can be separated from the solution and can provide a high-grade smelter feed for ferrochromium production.

It is essential for economical reasons to operate the process with a minimum of chromium being dissolved in the acid leach, particularly if the leach solution is to be wasted. It is also desirable to operate the process with a maximum amount of iron dissolved in the leach to obtain economic use of the acid or a maximum of iron dissolved per unit of sulfuric acid consumed in the process.

By careful control of the proportions of reduced chromite material to acid in the leach, having a pH value between 2.0 and 6.0 (preferably 4.0) in the final leach, the amount of chromium dissolved can be controlled or maintained at a low value and a high iron concentration can be maintained at the same time in the leach solution. At a pH below 6.0 (preferably 3.9 to 4.0) the iron salts are not precipitated as iron hydroxide, Whereas the chromium salts in solution are essentially precipitated (see Figures 2 and 3).

The published values for pH at which precipitation of ferrous and chromic ions occur as hydroxides do not show enough difference to suggest a good separation by precipitation of one from the other in aqueous, solution.

41 The present laboratory results, however, indicate that effective separation of the chromium and iron salts is obtained by precipitation of the chromium salts in presence of high iron concentration (Figalre 2 and Table I). It is presumed that the iron in solution is ferrous (divalent) sulfate and that the chromium in solution is chromous (divalent) or chromic (trivalent) sulfate. No hexa-valent chromium is present under the reducing conditions which exist in the'leach. If trivalent iron were present, it would be precipitated as the hydroxide in the pH range of this separation step, about 4.0. The chromium. is probably present as chromous (divalent) salt in solution during the leach; however, chromous salt is a strong reducing agent and in the presence of air, converts to chromic (trivalent) salt much more readily than ferrous salt converts to ferric salt. The chromous or chromic salt probably precipitates as the hydroxidein the pH range 3 to 4.5, while the ferrous salt is retained in solution. The presence of chromous salt would always insure that the dissolved iron be present in the ferrous state since chromous salts can react with ferric salts to reduce them to the ferrous state.

Experiments reported in part in Table I and Figure 7 4 have shown that the reduction-roasted chromite material can be leached in a single stage leach to dissolve over 50 percent of the iron content while dissolving less than 0.1 percent of the chromium to obtain a residue ratio, chromium to iron of 2.98, from roasted material or concentrates having a ratio, chromium to iron of 1.46, Figure 4. The final pH of the leach is controlled at about 4.0. However, it has been found preferable to conduct the leach countercurrently or in stages (two being sufiicient) Figure 5. A part of the roasted chromite material is leached with excess acid at high rates of reaction (pH final=1.5 to 2.9) and filtered. The

solution is reacted with controlled additions of freshly roasted chromite material to raise the pH to about 3.9. The chromium which was dissolved in the first stage is precipitated in the second stage and combined with the residue. The iron content of the second batch of roasted chromite material is partially dissolved at the same time. From the two stage leach, two residues having high chromium/iron ratios are obtained (the first residue has the higher ratio) and may be blended or used separately in combination with untreated chromite concentrates to furnish a smelter feed having ratios of chromium to iron equal to 3.0 or higher as desired. A countercurrent technique also can be used in leaching to obtain a single residue, but the pH control obtained by proportioning the roasted chromite to acid is necessary to obtain high iron concentration in solution and low chromium in solution, Figure6. With multiple stage leaching and pH control, the recovery of chromium from the roasted chromite material obtained in the residues can be over 99 percent, while at the same time, eflicient utilization of acid can result. Present results show that over 0.5 lb. of iron can be extracted per lb. of sulfuric acid (100% H used. Theoretical maximum for the iron extraction would be 0.57 lb. iron/lb. H 50 the values corresponding to ferrous sulfate. Very little of the other constituents of the ore or concentrate, such as MgO, SiO- A1 0 go into solution.

It has been found advantageous to dry, pelletize and sinter the residues, or various proportions of residue to other material, from the upgrading process to furnish a. smelter feed having suitable physical properties as well as desired chromium to iron ratios. These operations, however, are not considered essential to the invention.

Examples of the practice of this invention appear below by way of illustration but are not to be deemed as imposing limitations on the scope of the invention other,

pelletized. using; molasses in water molasses in water solution byvolume) Both; pcllctized and powdered mixes; were roastedj in, a; muffle; furnace for 2.5 hr. at 2200 F. in covered crucibles. The roasted material was e concen ra es th. especto. hromium amp s na grindnc: t e rQ nium-h t ng ma e a a. fin s a meetin sa d; ma e al h: a. c ona o s; reducing: agen under r du in condit on leachin he r as e mater a then pulverized; to. 100%,-

100 me h and 'e m 5 n; ulfuric acid. said u f caei i g, p esen in a as follows:

Samples of the roasted chromite were then leached in quan ity uflicien to mainta nth n l. pH o id ach between. about 3 9. and 4): whe eby o form a rela uely c ncentrat d ferrous. sulfa s lu ion. e ati e .1: oncen ated. ch o o s;- nd' hromic lf so u i ns Fe 10 while main aining; re a ly gh nc ntr ion o hr n h r s u o o ed; n epara ing h r sidue from. s id. l ach solutio he; method, of. claim-. wher in.- sa d leach o u ion a d said r sidue are. s parat d y filtra on; a d. r sidue dilute sulfuric acid at 85 6. according to the techniques is dried, pelletized, bl nde w th additional materials,

illustrated in Figures 4, 5, and 6. The results. are noted in Table 1 according to leach technique. Single stage leaching was used in tests 1, 4, 5, 6-, and 7'. Two-stage leachingwas used. in. test 2, and countercurrent, leaching was used in test3.

sin ered nd ther after: m ted A method f up ad ng.- e ro hrom um or s. and concentrates. with respect to chromium comprising: grind ing; he c r miumrina-ma er l o a. ta oast. ns saidmaterial; with. a. carbon eous; reducing agent TABLE I Chemical upgrading test results Leach Residue Roast Test Chro- N o. mite, Percent 7 Percent Percent Wt., Gm H2804 Volume, Time, Temp, Stages Final Weight, Percent Percent C Cr Reby Wt. ml. hr. 0. pH Gm. CrrOs Fe Peg cent covery 100 8. 6 200 0. 5 85 1 4. 0 90.3 39.0 8539 3.0 99. 9 770 10.0 1, 500 1 0 s5 2 3.9 $352 8 5 g 98.5 100 8.5 200 1 0 85 0.0. 4.0 89.3 39.1 6.70 4.0 99.0 100 16.0 600 0 5 85 1 1 1. 64. l 45. 6 4. 62 6. 75 83. 0 100 16. 0 500 O 5 85 1 1 1. 69.2 45. 8 4. 31 7. 90.0 100 16.0 500 0 5 85 1 1 1. 74.3 41.7 6. 48 4.17 88.0 100 16.0 500 0 6 85 1 1. 73.3 42.8 5.00 5. 86 89.0

1 No pH control was exercised. 1 Material was not pelletized.

It is seen that by means of control of the leaching technique with pH control, ratios of chromium to iron in residues up to 7.0 with recovery of chromium in the neighborhood of 99% and iron extraction of over 0.5 lb.

under reducing conditions at an elevated temperature; leaching said roasted material with excess sulfuric acid at a pH less than about 2.0 and thereafter filtering the pulp so formed; partially neutralizing the leach solution iron per lb. H 80 consumed may be attained. These so formed with additional roasted material until the pH are important economic factors and constitute substantial improvements over methods heretofore employed as such prior art methods have been unable to obtain consistent results-chromium to iron ratios in the neighborhood of said leach is raised to between about 2.0 and 6.0 whereby to precipitate dissolved chromous and chromic salts leaving said solution with a low chromium, as sulfate of chromium, concentration and to leave the iron material,

of 4.0 and, at the same time, a recovery of chromium 50 as ferrous sulfate, in solution while simultaneously disover 95%.

Obviously many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and theresolving additional iron from said additional roasted material; and separating the said leaching solution from the residue.

6. The method of claim 5 wherein said residue is fore only such limitations should be imposed as are indidried, pelletized, sintered and thereafter smelted.

cated in the appended claims.

I claim:

1. A method of upgrading ferrochromium ores and concentrates with respect to chromium comprising: grind- 7. A method of upgrading ferrochromium ores and concentrates with respect to chromium comprising: grinding the chromium-bearing material to a fine state; roasting said material with a carbonaceous reducing agent ing the chromium-bearing material to a fine state; roastunder reducing conditions at an elevated temperature;

ing said material with a carbonaceous reducing agent under reducing conditions; leaching the roasted material in sulfuric acid, said sulfuric acid being present in a quantity sufficient to maintain the final pH of said leaching said roasted material with excess sulfuric acid at a pH less than about 2.0 and thereafter filtering the pulp so formed; partially neutralizing the leach solution so formed with additional roasted material until the pH leach between about 2.0 and 6.0 whereby to form a relaof said leach is raised to between about 3.9 and 4.0

tively concentrated ferrous sulfate solution and relatively unconcentrated chromous and chromic sulfate solutions while maintaining a relatively high concentration of chromium in the residue so formed; and separating the residue from said leach solution.

2. The method of claim 1 wherein said leach solution and said residue are separated by filtration; said residue is dried, pelletized, blended with additional materials, sintered and thereafter smelted.

3. A method of upgrading ferrochromium ores and 75 whereby to precipitate dissolved chromous and chromic salts leaving said solution with a low chromium, as sul fate of chromium, concentration and to leave the iron material, as ferrous sulfate, in solution while simultaneously dissolving additional iron from said additional roasted material; and separating the said leaching solution from the residue.

8. The method of claim 7 wherein said residue is dried, pelletized, sintered and thereafter smelted.

9. A method of upgrading fcrrochromium ores and concentrates with respect to chromium comprising: grinding said chromium-bearing material to a fine state; roasting said'chromium-bearing material with a carbonaceous reducing agent'under reducing conditions at an elevated temperature; leaching the roasted material so formed with sulfuric acid in a countercurrent leach solution wherein 'said roasted chromium material is moved countercurrent to said leach solution, said solution being maintained at a pH within the range of about 2.0 to 6.0 whereby to form a leach solution containing a relatively high iron concentration and a relatively low chromium concentration, and a residue containing a relatively high chromium concentration and a relatively low iron concentration and thereafter separating the solids from the liquid whereby to form a residue high in chromium. I 10. The process of claim 9 wherein the final leach pH is maintained betweenabout 3.9 and 4.0 whereby to form a final leach solution containing relatively large quantities of iron and relatively small quantities of chromium with relatively high chromium recovery in the residue and relatively low iron concentration in said residue.

11. The process of claim 10 wherein said residue is '38 separated from said liquid, dried, pelletized, blended with additional materials, sintered and thereafter smelted.

37 12. The process of claim 13 wherein the pH is adjusted between about 3.9 and 4.0 Y I 13. In a process wherein an ore containing iron and chromium is reduction-roasted to form a roasted material containing reduced iron and the roasted material so formed is treated with aqueous sulfuric acid to dissolve iron and chromium salts from said reduced material, the

improvements comprising: employing said aqueous sulfuric acid in such a quantity and concentration that the References Cited in the file of this patent UNITED STATES PATENTS 9,853 Booth July 19, 1853 

1. A METHOD OF UPGRADING FERROCHROMIUM ORES AND CONCENTRATES WITH RESPECT TO CHROMIUM COMPRISING: GRINDING THE CHROMIUM-BEARING MATERIAL TO A FINE STATE; ROSTING SAID MATERIAL WITH A CARBONACEOUS REDUCING AGENT UNDER REDUCING CONDITIONS; LEACHING THE ROASTED MATERIAL IN SULFURIC ACID, SAID SULFURIC ACID BEING PRESENTS IN A QUANTITY SUFFICIENT TO MAINTAIN THE FINAL PH OF SAID LEACH BETWEEN ABOUT 2.0 AND 6.0 WHEREBY TO FORM A RELATIVELY CONCENTRATED FERROUS SULFATE SOLUTION AND RELATIVELY UNCONCENTRATED CHROMOUS AND CHROMIC SULFATE SOLUTIONS WHILE MAINTAINING A RELATIVELY HIGH CONCENTRATION OF CHROMIUM IN THE RESIDUE SO FORMED; AND SEPARATING THE RESIDUE FROM SAID LEACH SOLUTION. 